THIS invention relates to an isotactic polypropylene based composite and more particularly to the production of the highly nucleating isotactic polypropylene composite.
Polypropylene (PP) is one of the most widely used commodity polymers because of its excellent cost-performance value. However, PP has a low upper service temperature [1], which can be correlated with the heat distortion temperature (HDT) and Vicat softening point. One way to address this drawback is to introduce cross-linking between polymer chains because cross-linking leads to a polymer matrix with higher thermal and thermo-mechanical stabilities [1]. In the case of PP, the chemical inertness makes functionalization and cross-linking very difficult. One way to cross-link PP chains is via peroxide reactions followed by silane-water cross-linking. Free radicals are usually generated during peroxide reactions in the first step. In the second step, the silane-grafted polymer is cross-linked via exposure to a humid environment [2]. However, both steps are tedious and not commercially viable.
Another potential approach is the incorporation of additives, such as inorganic fillers [3-6], cellulose [7], natural fibers [8], etc. Several studies have been conducted on the effect of additives on the thermo-mechanical properties of PP. In most cases, surface treated fillers were used to improve the interfacial interaction between the polymer matrix and filler surface. For example, to improve the dispersion of glass fibers in the PP-matrix, it was necessary to modify the glass fiber surface with γ-aminopropyltriethoxysilane [3]. So far, the greatest improvement in HDT of PP was achieved with 50% cellulose in the presence of maleic anhydride-grafted PP (PP-g), and it is approximately 66° C. higher than that of neat PP (MFI=75 g/10 min, measured at 230° C. and 2.16 kg) [7]. Therefore, by adding fillers, the HDT of PP can be improved to a certain extent.
Another challenge with the PP matrix is its lower crystallization temperature. In the molding industry, the cycle time is dependent on the crystallization temperature of the polymeric material. Therefore a polymeric material having a low crystallization temperature requires a longer cycle time in the molding process. As a result more energy is required and which may become costly.
Cross-linking or the introduction of a network structure in a polymer matrix usually imposes restriction on the macromolecular chain mobility. Hence, the cross-linked material can stay rigid and exhibit dimensional stability [9]. In this direction, the restriction on the polymer chain mobility can be obtained by incorporation of filler which in turn also enhances the HDT of the neat polymer matrix [8]. The improvement in the HDT of the PP matrix also depends on other factors, such as crystallinity, crystal size [10], β-crystal formation [11], orientation of crystals in the presence of a nucleating agent [12], and relaxation and recrystallization during thermal annealing [13].
Sirisinha et al. [14] used the silane grafting and water cross-linking method to prepare a PP-based composite with stearic acid-coated calcium carbonate. They found that, besides silane grafting, the addition of stearic acid coated calcium carbonate on top of silane grafting can improve the HDT of PP (MFI=3.64 g/10 min) by 80° C. For this reason, they prepared a master-batch of PP and stearic acid-coated calcium carbonate. Then, the master-batch was tumble-mixed with vinyltrimethoxysilane and dicumyl peroxide solution and kept in a nitrogen environment overnight. Subsequently, the mixture went under further extrusion and molding processes prior to the water crosslinking step. However, this process is tedious and not energy efficient; therefore, it is not commercially viable.
U.S. Pat. No. 4,146,529 Yamamoto et al. [15] discloses reaction of an endo-bicyclo[2.2.1]-5 heptine-2,3dicarboxylic anhydride-modified PP with an amino or epoxy silane. The purpose of this reaction was to use the alkoxy groups to bind the fillers and to react with the non-grafted carboxylic anhydride to form low odor and non-volatile products [15]. A very fast reaction between aminosilane and the grafted acid anhydride was also reported in the European patent EP 1 021 486 B1 on silane vulcanized thermoplastic elastomers [9]. In this case, the carboxylic anhydride was grafted on the rubber phase polymer (elastomer). The authors of EP 10 21 486 B1 used SILQUEST A-186 [γ-(3,4-epoxycyclohexyl) ethyltrimethoxy-silane], SILQUEST A-187 (γ-glycidoxypropyl-trimethoxysilane) and SILQUEST A-189 (γ-mercaptopropyltrimethoxysilane), and studied the effect of mixing sequence on the crosslinking and mechanical properties; however, there were no improvement in the HDT. To overcome this challenge, the authors first blended all of the ingredients for 5 min and then added the thermoplastic polymer and silane. In another process, all of the ingredients were mixed at the same time. However, the end product originating from their reaction mechanism was not a stable one (see, for example, FIG. 1).
Over the last few years, several nucleating agents including nanoclay have been used to improve the crystallization temperature (Tc) of PP. Liu et al. [16] reported that in the presence of co-intercalated organoclay (hexadecyltrimethyl-ammonium and epoxypropyl methacrylate-modified montmorillonite, MMT), the crystallization temperature of PP can be improved by 12° C. In presence of PP-g and C15A (commercially available organoclay, Cloisite® 15A), an improvement of 7° C. in Tc can be achieved [17]. The same level of improvement can also be achieved with C20A (commercially available organoclay, Cloisite® 20A) [18]. In the presence of the nucleating agents, such as cis-calcium hexahydrophthalate and disodium bicyclo[2.2.1]heptane-3-dicarboxylate, the Tc of syndiotactic PP improved by 17° C. and 11° C., respectively [19]. However, such nucleating agents might not reduce the odor of MA in PP-g.
It is accordingly an object of the invention to provide an isotactic polypropylene based composite that will, at least partially, alleviate the above disadvantages.
It is also an object of the invention to provide an isotactic polypropylene based composite which will be a useful alternative to existing isotactic polypropylene based composite.